Asynchronous Transfer Mode (ATM) or "cell switching" is a method of transmitting digital information wherein the information is broken into equal sized units called cells. The individual cells of information are transmitted from a source node to a destination node through a pathway (or connection) through a digital network. The digital network may be constructed of digital switches coupled together by digital communication links which carry the cells of information between the digital switches along the connection. The digital switches route the cells from incoming communication links to outgoing communication links and finally to the destination node.
Before any information can be transported through the network as ATM cells, the information must be segmented into cells. Typically, this is accomplished by segmentation and reassembly (SAR) engines which are used to "chop up" user data (typically formatted as variable length frames) into fixed length cells for transmission. As the data frames are segmented, ATM header information is added to each cell and the cells are transmitted into the ATM network. During reception, the header information is stripped from the cells and the remaining fixed length payloads are reassembled into variable length data frames for use by appropriate user applications. Typically, the payload information for each cell is copied into memory as it is received during the reassembly process.
In ATM networks the segmentation and reassembly (SAR) functions are performed at the interface between an ATM layer and an ATM adaptation layer (AAL). During transmission, higher level services pass user data to the AAL where, first, a convergence sublayer forms convergence layer protocol data units (CS-PDUs). These CS-PDUs will, generally, be larger than the payload capacity of an ATM cell. Therefore, a second sublayer, the segmentation and reassembly (SAR) sublayer, accepts the variable length CS-PDUs and fragments them into appropriate payloads (48-bytes) for transmission. These payloads are referred to as segmentation and reassembly protocol data units (SAR-PDUs) and are passed to lower layers in the ATM protocol model for further processing. During reception, a reverse operation takes place and the SAR sublayer is responsible for reassembling messages as they are received.
Several ATM AALs have been developed for various applications. AAL Type 5 (AAL-5) was designed for variable bit rate, connection-oriented or connectionless services and provides good error detection/correction capabilities and high line efficiency. The AAL-5 methodologies are illustrated in FIG. 1. During transmission, a user message 10 is passed from higher level protocols to the AAL, where it is referred to as an AAL Service Data Unit (AAL-SDU) 12. As shown in FIG. 1, the AAL-5 convergence sublayer uses the AAL-SDU 12 to create variable length protocol data units (CS-PDUs) 14. The CS-PDUs are made up of various fields. The User Data Field 16 comprises data from the higher layer protocols and may be up to 65,535 bytes in length. The Pad field 18 is used to align the entire CS-PDU 14 on a 48-byte boundary (i.e., the entire CS-PDU 14 is a multiple of 48-bytes in length). The Control field 20 is two bytes in length and is presently reserved for future use. The Length field 22 indicates the actual length of the User Data Field 16 and is two bytes long. The CRC-32 field 24 is appended to provide error detection in the CS-PDU 14.
CS-PDUs are passed to the AAL-5 SAR sublayer. The SAR sublayer fragments the CS-PDUs to fixed-length SAR-PDUs 26, each 48-bytes long. There are no new headers or trailers added by the SAR sublayer in AAL-5. No padding is necessary because the CS-PDU 14 was padded to a multiple of 48-bytes. As further shown in FIG. 1, the SAR-PDU 26 is passed to the ATM layer where a 5-byte header 28 is added to form an ATM cell 30. During reception, these operations are performed in a reverse order to rebuild the user message 10.